Background:As members of the class Amphibia,
frogs
may live some of their adult lives on land, but they must return to water to
reproduce. Eggs are laid and fertilized in water. On the outside of the frog’s
head are two external nares, or nostrils; two
tympani, or
eardrums; and two eyes, each of which has three lids. The third lid, called the nictitating
membrane, is transparent. Inside the mouth are two internal nares, or
openings into the nostrils; two vomerine teeth
in the middle of the roof
of the mouth; and two maxillary teeth at the sides of the mouth. Also
inside the mouth behind the tongue is the pharynx,
or throat.

In the
pharynx, there are several openings: one into the esophagus, the tube
into
which food is swallowed; one into the glottis,
through which air
enters the larynx, or voice box; and two into the
Eustachian tubes, which
connect the pharynx to the ear. The digestive system consists of the organs of
the digestive tract, or food tube, and the digestive
glands. From the esophagus,
swallowed food moves into the stomach and then into the
small intestine. Bile is a digestive juice made by the
liver and stored in
the gallbladder. Bile flows into a tube called the
common bile duct, into
which pancreatic juice, a digestive juice from the
pancreas, also
flows. The contents of the common bile duct flow into the small intestine, where
most of the digestion and absorption of food into the bloodstream takes place.

Indigestible materials pass through the
large intestine and then into the cloaca, the common exit chamber of the digestive, excretory, and
reproductive systems. The respiratory system consists of the
nostrils and the larynx, which opens into two
lungs, hollow sacs with thin walls. The
walls of the lungs are filled with capillaries,
which are microscopic
blood vessels through which materials pass into and out of the blood. The
circulatory system consists of the heart, blood vessels, and
blood. The heart
has two receiving chambers, or atria, and one sending chamber, or
ventricle. Blood is carried to the heart in vessels called
veins. Veins from different
parts of the body enter the right and left atria. Blood from both atria goes
into the ventricle and then is pumped into the arteries, which are blood
vessels that carry blood away from the heart.

The urinary system consists of the
frog’s kidneys, ureters, bladder, and cloaca. The
kidneys are organs
that excrete urine. Connected to each kidney is a
ureter, a tube through
which urine passes into the urinary bladder,
a sac that stores urine
until it passes out of the body through the cloaca. The organs of the male
reproductive system are the testes, sperm ducts, and
cloaca. Those of the female
system are the ovaries, oviducts, uteri, and
cloaca. The testes produce sperm,
or male sex cells, which move through sperm ducts,
tubes that carry
sperm into the cloaca, from which the sperm move outside the body. The ovaries
produce eggs, or female sex cells, which move through oviducts into
the uteri, then through the cloaca outside the body.

The central nervous
system of the frog consists of the brain, which is enclosed in the
skull,
and the spinal cord, which is enclosed in the
backbone. Nerves branch out from
the spinal cord. The frog’s skeletal and muscular systems consist of its
framework of bones and joints, to which nearly all the
voluntary muscles of the
body are attached. Voluntary muscles, which are those over which
the frog has control, occur in pairs of flexors and
extensors. When a flexor of
a leg or other body part contracts, that part is bent. When the extensor of that body part contracts, the part straightens.

Objectives:• Describethe
appearance of various organs found in the frog.• Namethe organs that make up various systems of the frog.

Purpose:In this lab, you will dissect a frog in
order to observe the external and internal structures of frog anatomy.

Place
a frog on a dissection tray. To determine the frog’s sex, look at the
hand digits, or fingers, on its forelegs. A male frog usually has thick pads on
its "thumbs," which is one external difference between the sexes, as shown
in the diagram below. Male frogs are also usually smaller than female frogs. Observe
several frogs to see the difference between males and females.

Use
the diagram below to locate and identify the external features of the head. Find
the mouth, external nares, tympani, eyes, and nictitating
membranes.

Turn
the frog on its back and pin down the legs. Cut the hinges of the mouth and open
it wide. Use the diagram below to locate and identify the structures inside the
mouth. Use a probe to help find each part: the vomerine
teeth, the maxillary
teeth, the internal nares, the tongue, the openings to the Eustachian
tubes, the esophagus, the pharynx, and the slit-like glottis.

Look
for the opening to the frog’s cloaca, located between the hind legs. Use
forceps to lift the skin and use scissors to cut along the center of the body
from the cloaca to the lip. Turn back the skin, cut toward the side at each leg,
and pin the skin flat. The diagram above shows how to make these cuts

Lift
and cut through the muscles and breast bone to open up the body cavity. If your
frog is a female, the abdominal cavity may be filled with dark-colored
eggs. If
so, remove the eggs on one side so you can see the organs underlying them.

Use the diagram below to
locate and identify the organs of the digestive system: esophagus, stomach,
small intestine, large intestine, cloaca, liver, gallbladder, and pancreas.

Again
refer to the diagram below to identify the parts of the circulatory and
respiratory systems that are in the chest cavity. Find the left atrium, right
atrium, and ventricle of the heart. Find an artery attached to the
heart and another artery near the backbone. Find a vein near one of the
shoulders. Find the two lungs.

Use
a probe and scissors to lift and remove the intestines and liver.
Use the diagram on the next page to identify the parts of the urinary and
reproductive systems. Remove the peritonealmembrane, which is
connective tissue that lies on top of the red kidneys. Observe the yellow fatbodies that are attached to the kidneys. Find the ureters; the urinary
bladder; the testes andsperm ducts in themale; and theovaries,
oviducts, and uteri in the female.

Remove
the kidneys and look for threadlike spinal nerves that extend from
the spinal cord. Dissect a thigh, and trace one nerve into a leg
muscle.
Note the size and texture of the leg muscles.

Dispose
of your materials according to the directions from your teacher.